Destruction of a crypto asset to decrease supply.
This is usually achieved by sending the coins to a burn address. An address no one has has access or the private keys. Burn addresses usually have a large amount of repeated characters and don't look like standard addresses - usually known addresses with a long string of repeating characters.
Coins and tokens can also be un-intentionally burned if they are used with an invalid transaction, or the to address is malformed.
If access to some coins are lost due to losing the passphrase, death, etc, then those coins are effectively burnt as well.
* All terms and definitions may update as the Cryptionary improves.